This invention relates to a recording method of medical image and a medical image recording apparatus capable of producing a medical image which has such a high quality as to be fit for diagnosis as a transmission image and is of such a good image quality that it never produces reversing of density gradation when it is observed as a reflection image.
In recent years, in X-ray radiography, in place of an SF system, a system for picking up a digital electrical signal of an X-ray image such as a computed radiography (CR) system or a system employing a flat panel X-ray detector (FPD) has appeared. With the spreading of what is called a digital X-ray image pickup apparatus, also a digital medical-use image recording apparatus for recording a medical image on the basis of an electrical signal obtained by a CR system or an FPD system is spreading.
A recording method which has now become the greatest mainstream is a silver halide laser writing method in which an image is formed through converting an electrical signal of an X-ray image obtained by a CR system or an FPD system into laser beam intensity variation and carrying out print and development processing on a conventional silver halide film. However, because the method uses a silver halide film in the same way as a conventional method, there is a problem that it is troublesome and costs much.
As regards a method not using a silver halide film, a thermal transfer method or a sublimation-type printer can be considered. However, in the case of a thermal transfer method, the ink of a recorded image is present on the uppermost surface of a film, which produces a trouble such that ink is easy to be transferred in handling. Further, in the case of a sublimation-type printer, sufficient density cannot be obtained and waste matter such as an ink ribbon is produced after image formation as in the case of a thermal transfer method.
Lately, an image recording apparatus employing an ink jet method has become versatile as a small-sized low-priced printer, which enables the great improvement of the resolution and quality of a recorded image. Therefore, by applying an ink jet recording apparatus to X-ray image formation, the above-mentioned trouble is to be solved, and it is expected that an ink jet image forming method capable of forming an X-ray image which is made of low cost and easy to observe by making the most of the advantage of an ink jet printer can be provided.
Now, it is a subject in an image recording apparatus of not only an ink jet method but also all other recording methods that, for a medical image used mainly in diagnosis, an extremely high image quality is required.
It is said that the number of gray levels in a simple X-ray radiograph required for diagnosis is 10 bits (=1024 gray levels), and further, the number of gray levels enabling sufficient diagnosis is 12 bits (=4096 gray levels). In the case where an image of multiple gray levels such as a medical image is expressed by an ink jet method, because the number of ink density levels is limited, it is necessary to make the gradation expression of a recorded image in a digital way. For example, there is a method in which one pixel of image data is composed of a matrix having a plurality of elements, for example, a dither matrix of 4xc3x974 elements, and gradation expression of 4xc3x974+1=17 gray levels is achieved by using so called a dither method with this dither matrix made a unit.
Further, by using a plurality of kinds of ink, for example 4 kinds of ink, having colors of the same hue but different densities respectively, the number of gray levels to be produced can be increased innumerably. However, actually it is general that gradation expression is made on the basis of an error diffusion method by selecting several to several tens of dither matrices out of all the dither matrices that are able to be produced and utilizing these several to several tens of dither matrices. As regards the literatures concerning an error diffusion method, for example, it is described in detail in xe2x80x98R. FLOYD and L. STEINBERG, xe2x80x9cAN ADAPTIVE ALGORITHM FOR SPATIAL GRAY SCALExe2x80x9d, SID 75 DIJEST, pp. 36 to 37xe2x80x99. By using this method composed of a dither method combined with an error diffusion method, multiple gray scale expression of 12 bits is possible, and by selecting suitable dither matrices and using a suitable error diffusion algorithm, it is possible to obtain a smooth gradation characteristic.
However, the above-mentioned error diffusion method is what is called an area-modulation method, and has the defect that it makes the roughness (noise) of an image larger as the compensation against the advantage that it is capable of expressing multiple gray levels. Therefore, it is necessary to increase the number of dither matrices to the utmost, but because the number of densities of the ink having the same hue is limited, it is often used a method in which the number of gradations is increased by shooting ink drops of different densities approximately at the same position.
As described in the publication of the examined patent application H5-46744, it has been known an invention utilizing the fact that the reflection density in the case where high-density dots and low-density dots are formed in a superposed way at the same cell in a dot matrix is different from that in the case where high-density dots and low-density dots are formed at different cells respectively in the above-mentioned dot matrix. Further, there is a method in which multiple gray level expression is made by utilizing positively the difference in the recording density to be produced by varying the order of superposing as keeping constant the number of the high-density and low-density dots.
As described in the publication of the unexamined patent application H3-218851, there is a method in which recording is done first with high-density ink and successively with low-density ink superposed. It is a method utilizing the nature such that, in a reflection image, in the case where recording is made with low-density ink followed by high-density ink, it appears as if a large dot of high-density ink is shot, which increases graininess, but in the case of high-density ink followed by low-density ink, a dot appears not so large as that in the former case.
It has been known that the above-mentioned two methods are appropriate recording methods for recording a reflection image, but the effect of the above-mentioned methods could not be obtained for a transmission image. On the contrary, in the case where an image is recorded by an ink jet method, they have rather the defect that the gradation is reversed in a part of an image in accordance with the order of shooting ink drops of different densities. It is considered that a transmission image has a characteristic which is proper to a transmission image and there is a method appropriate for the recording of a transmission image which is different from that of recording a reflection image.
The phenomenon that gradation is reversed is a problem peculiar to an ink jet method; however, in the first place, to use both a transmission image and a reflection image has been regarded as difficult in various image recording methods such as a silver halide method, a thermal transfer method, and other methods. Among various reasons which can be cited, the most difficult reason is the difference in the density characteristic between a transmission image and a reflection image.
FIG. 5 is a drawing showing a typical reflection density vs. transmission density characteristic. The detail is described in xe2x80x98Yasushi Ohyama, xe2x80x9cThe relation between transmission density and reflection density of a photographic image layerxe2x80x9d, Journal of Japan Photographic Society, 41(1), pp. 42 to 59 (1978)xe2x80x99. As regards this characteristic, it is not limited to a silver halide photography, but a similar tendency can be observed also in a recording apparatus of an ink jet method. The reason is that the structure of the recording medium is hardly different between ink jet recording and silver halide photographic recording, and the difference is only that silver particles as the image forming element are substituted by a dye material or a pigment material.
In cases where an image is recorded by a conventional ink jet method, recording is made separately for a transmission medium and for a reflection medium in most cases; therefore, it has been necessary to prepare respective gradation tables for expressing a gradation of a reflection image and a transmission image. The greatest reason for the incompatibility of a gradation table between a reflection image and a transmission image is that the above-mentioned reflection vs. transmission density characteristic is non-linear, which causes the gradation characteristics of both to appear different. Up to now, in accordance with use, it has usually been put in practice that either a transmission image or a reflection image was formed by an image forming method that is suitable to one or the other.
However, in some cases it is very convenient to use one sheet of a recorded image either way in accordance with the purpose of use. For example, in the case where a recorded medical image is used as it is for a patient""s chart, diagnosing can be done by using it as a transmission image, and even in the case where a light box is not provided near by, it can be used for the explanation to the patient as a reflection image; this is convenient.
However, if an image which has been recorded by using a gradation method for a transmission medium is observed as a reflection image, there is a high possibility of producing the gradation reversing which is peculiar to an ink jet method; this is a problem. There is also a risk to cause the diagnosis to become erroneous if it is used as a reflection image through an error. Further, if an image which has been recorded by using a gradation method for a reflection medium is observed as a transmission image, there is a high possibility of producing this reversing of gradation in the same way, and there is a risk to cause the diagnosis to become erroneous if it is used as a transmission image through an error; this is also a problem.
This invention has been made in view of the above-mentioned actual situation, and it is its object to provide a recording method of medical image and a medical image recording apparatus capable of producing a medical image which has such a high quality as to be fit for diagnosis as a transmission image and is of such a good image quality that it never produces reversing of density gradation when it is observed as a reflection image.
For the purpose of solving the above-mentioned problems and accomplishing the object, this invention has the structures described below.
[Structure 1]
A recording method of a medical image comprising:
jetting ink onto a recording medium to make the medical image,
wherein a reflection density vs. transmission density characteristic curve of the recorded medical image is monotone non-decreasing in the range of the transmission density being not more than 2.0.
[Structure 2]
The recording method of Structure 1, wherein the recording medium has a transmission density of 0.15 to 0.40.
[Structure 3]
The recording method of Structure 2, wherein a visual density VD of the recording medium and a blue light density BD of the recording medium satisfy the following relation:
BD/VDxe2x89xa70.25. 
[Structure 4]
The recording method of Structure 1, wherein the difference between the transmission density and the reflection density of the same point of the recorded medical image is not more than 1.0 in the range of the transmission density being not more than 2.0.
[Structure 5]
The recording method of Structure 4, wherein the difference between the transmission density and the reflection density is not more than 0.7.
[Structure 6]
The recording method of Structure 4, wherein, in the jetting step, a plurality of ink drops are jetted onto substantially same point of the recording medium.
[Structure 7]
The recording method of Structure 6, wherein the medical image is recorded in a resolution of not less than 360 dots/25.4 mm.
[Structure 8]
The recording method of Structure 1, wherein a slope of tangent line of a point in the range of the transmission density being not more than 1.0 on the reflection density vs. transmission density characteristic curve is not more than 2.3.
[Structure 9]
A recording method of a medical image comprising: jetting ink onto a recording medium to make the medical image,
wherein the difference between a transmission density and a reflection density of the same point of the recorded medical image is not more than 1.0 in the range of the transmission density being not more than 2.0.
[Structure 10]
The recording method of Structure 9, wherein the difference between the transmission density and the reflection density is not more than 0.7.
[Structure 11]
The recording method of Structure 9, wherein, in the jetting step, a plurality of ink drops are jetted onto substantially same point of the recording medium.
[Structure 12]
The recording method of Structure 11, wherein the medical image is recorded in a resolution of not less than 360 dots/25.4 mm.
[Structure 13]
An apparatus to recording a medical image onto a recording medium by the recording method described in claim 1, the apparatus comprising a plurality of recording heads being capable of jetting a plurality of inks having the same color hue and different concentrations from each other,
wherein the recording heads are arrayed in the approximately perpendicular direction to the conveying direction of the recording medium, and in the order of the concentration of the ink in the recording head.
[Structure 14]
The apparatus of Structure 13, wherein the recording heads move backwards and forwards in the approximately perpendicular direction to the conveying direction of the recording medium, the recording heads jet the ink on one way of the backwards movement and the forwards movement, and the recording head are arrayed in descending order of the concentration of the inks in the recording heads along the way, on which the ink is jetted.
By this invention, it is possible to make the reversing of density not occur particularly in low to medium density region in a reflection density vs. transmission density characteristic curve. Further, in a reflection density vs. transmission density characteristic curve, by limiting the gradient of the characteristic curve in the low to medium density region, it is possible not to produce a large difference in gradation between transmission image and reflection image. Moreover, it is desirably used a method in which a plurality of ink drops are shot approximately at the same position repeatedly, reflection density is controlled by combining the order of jetting and the densities of used ink, and a low-density ink drop is shot last.
In this way, an image having a high image quality as a transmission image or also as a reflection image can be produced.
The term xe2x80x9cdensityxe2x80x9d used in the invention represents what is called an optical density D, and is defined by Dt=xe2x88x92log10T or Dr=xe2x88x92log10R. It means diffuse light density measured by, for example, an optical densitometer PDA-65 (manufactured by Konica Corp.). In addition, T and R denote light transmittance and light reflectance respectively; the former density Dt is one called transmission density, and the latter density Dr is one called reflection density. Because the invention can be applied to both densities of transmission density and reflection density, the term density represents either of the transmission and reflection density unless otherwise specified. Further, the term xe2x80x9cimage densityxe2x80x9d means a density which an image has, and represents an overall image density including the density caused by a recording material (such as ink) adhering to a recording medium and the density caused by the recording medium.
The term xe2x80x9ctransmission recording mediumxe2x80x9d used in the invention means a recording medium to be used mainly for observing an image as a transmission image, and the term xe2x80x9creflection recording mediumxe2x80x9d used in this invention means a recording medium to be used mainly for observing an image as a reflection image. The term xe2x80x9ctransmission imagexe2x80x9d means an image to be observed in a form of transmission image observation. In the form of transmission image observation, an assistant light-source, which has a capability of emitting a high-intensity light, is provided on the backside of the image, and the transmission light, which is emitted from the assistant light-source and transmits the image, is used for observing the image. The assistant light-source provided on the backside of the image is usually referred a backlight. The term xe2x80x9creflection imagexe2x80x9d means an image to be observed in a form of reflection image observation. In the form of reflection image observation, an assistant light-source is provided at the front of the image, and the reflection light, which is emitted from the assistant light-source and is reflected by the image, is used for observing the image.
The term xe2x80x9ca density gradation characteristicxe2x80x9d used in the invention means a characteristic showing the relation between the signal value (abscissa) in an image signal and the density (ordinate) of the image recorded on a recording medium on the basis of the signal value, and an image recording apparatus records an image on a recording medium on the basis of this density gradation characteristic.
The term xe2x80x9ca reflection density vs. transmission density characteristic curvexe2x80x9d used in the invention means a characteristic showing the relation between the transmission density (abscissa) and the reflection density (ordinate) of a recording medium recorded thereon an image on the basis of a specified signal value. The term xe2x80x9cmonotone non-decreasingxe2x80x9d of a reflection density vs. transmission density characteristic curve represents a characteristic such that the reflection density does not decrease with the increase of the transmission density (that is, the reflection density increases or keeps a constant value) in a predetermined range of the transmission density.
Visual Density (VD) in this invention represents a diffuse transmission light, which satisfies the Spectral Condition of the visual density defined in ISO 5/3-1995 (Spectral Condition of the density measurement) and the Geometrical Condition defined in ISO 5/2-1984 (Geometrical Condition of the transmission density measurement), and it is conventionally used in the art. Blue Light Density (BD) in this invention represents a blue light density, which satisfies the Spectral Condition of xe2x80x9cStatus Axe2x80x9d defined in ISO 5/3-1995 and the Geometrical Condition defined in ISO 5/2-1984. The term xe2x80x9cBD/VDxe2x80x9d used in this invention means the ratio of the blue light density (BD) to the visible light density (VD). For example, in the case where an optical densitometer PDA-65 (manufactured by Konica Corp.) is used, BD/VD can be obtained by measuring the density with an amber filter for VD and a blue filter for BD used.
The term xe2x80x9cresolution of an image recording apparatusxe2x80x9d means an index represents the recording density of an image recording apparatus, and the unit dpi (dots per inch: 1 inch=about 25.4 mm) is generally used. For example, at a resolution of 360 dpi, the minimum recording size to be controlled by the image recording apparatus (hereinafter referred to as xe2x80x9cthe minimum recording sizexe2x80x9d) is equivalent to 25400 xcexcm/360 dpi=about 70 xcexcm. Further, the term xe2x80x9can output pixel sizexe2x80x9d means an output size corresponding to one pixel in an image signal, and it satisfies at least the relation (output pixel size)xe2x89xa7(minimum recording size).